What is influenza?

A respiratory illness associated with infection by the influenza virus. Symptoms include headache, fever, cough, sore throat, aching muscles and joints. There is a wide spectrum of illness ranging from minor symptoms through to pneumonia and death. Influenza is caused by a virus, which was first identified in 1933. There are two main types that cause infection, influenza A and influenza B (Influenza C is an uncommon type that infrequently causes infection). Influenza A is usually a more severe infection than influenza B. 

What does genetic drift and shift of influenza mean?

The influenza virus is antigenically unstable and new strains and variants are constantly emerging. Each year one or two subtypes of influenza A may be in circulation and one type of influenza B.    

Antigenic drift  – Minor changes to the amino acid sequence of the haemagglutinin (HA) molecules in the virus envelope take place all the time and cause genetic drift. Haemagglutinin is the main antigen associated with immunity. Neuraminidase (NA), the second main antigen in the virus plays a minor role in immunity. The drifted strains of influenza may infect partially immune people who have been exposed in previous winters. Influenza A drifts more than influenza B.   

Antigenic shift  – Genetic shift occurs when major changes in the HA or NA take place and a virus emerges which contains a haemagglutinin different from those of previously circulating viruses. When this happens it gives rise to major epidemics or pandemics in populations throughout the world that have no immunity to the new strains, e.g. Spanish flu (1918), Asian flu (1957) and Hong Kong flu (1968/69). Influenza virus strains are classified according to the place and year in which they were first isolated. Many epidemics of influenza originate in South East Asia and are thought to be due to rare recombination events between human, avian and other animal strains of influenza following coinfection in a susceptible host.  

How serious is influenza infection?

Influenza makes people feel worse than an ordinary cold. For most people influenza infection is just a nasty experience, but for some it can lead to illnesses that are more serious. The most common complications of influenza are bronchitis and secondary bacterial pneumonia. These illnesses may require treatment in hospital and can be life threatening especially for the older people, asthmatics and those in poor health. The influenza virus does not necessarily cause high mortality, but for old sick people it may speed up their death. During a pandemic, though, influenza can cause serious illness in young healthy individuals.

What are the symptoms of influenza?

The most common symptoms of influenza are an abrupt onset of fever, shivering, headache, muscle ache and dry cough. Most people confuse influenza with a heavy cold, however influenza is usually a more severe illness than the common cold, which is caused by other respiratory viruses. 

What are the symptoms of the common cold?

Cold symptoms are limited to the upper respiratory tract with runny nose, sneezing, watery eyes and throat irritation. The symptoms usually occur gradually and do not cause a fever or body aches.

When does influenza occur?

Influenza occurs most often in the winter months and usually peaks between December and March in the northern hemisphere. Illnesses resembling influenza may occur in the summer months but they are usually due to other viruses.

Why do people get infected with influenza during winter?

In temperate climates influenza strikes from late autumn through to spring, although technically influenza is not bound by seasons, and can occur all year round in tropical climates. A possible explanation for the high influenza virus activity in the wintertime is that people congregating indoors during winter facilitate the transmission of the virus or that more humid air indoors may help the viruses survive longer.

How is influenza diagnosed?

Usually, a doctor will diagnose a case of the flu based on typical symptoms of fever, chills, headache, cough and body aches. Specific lab tests to confirm the flu are costly and time consuming and are usually limited to outbreak or disease surveillance efforts.  

How is influenza spread?

The flu virus is highly contagious and is easily passed on by breathing in the tiny droplets from the breath of infected people. The incubation period – delay between infection and the appearance of symptoms – is about two – three days. The infectious period varies but an infected person can probably pass on the disease the day before their symptoms appear and remain potentially infectious for 3-5 days.

What should you do if you get flu?

Rest, drink plenty of fluids and take analgesics (paracetamol for all ages, aspirin may be taken by adults). Most influenza-like illnesses are self-limiting and may be caused either by influenza or other viruses/pathogens. It is best to treat the infection at home until the person is well enough to return to normal activities. Medical advice should be sought if symptoms become severe or last more than about a week. Those with chronic or long-standing illness may need medical attention earlier.

Who is most at risk from the complications of influenza?

The young have a greater risk of being infected because they have not developed immunity to the virus. Older people have a greater risk of the severe complications of infection such as pneumonia, because they often have underlying diseases, which reduce their resistance to infection. The immune response may also be less effective in older persons. The high-risk groups include individuals whose respiratory, cardiac or immune systems make them more vulnerable to flu and more likely to suffer severe illness.    

What precautions should people take?

Routine flu vaccination offers the best protection and people who are at high risk of infection should be vaccinated. It is difficult to avoid infection if there is an epidemic. Keeping away from crowded places can reduce the risk of becoming infected and spreading it to others. A previous flu infection or vaccination will not necessarily provide protection against further infections because the virus is continually changing genetically and different subtypes circulate each winter.

Source: Health Protection Agency